Internal combustion engines are a major contributor to harmful emissions. Internal combustion engines dominate land transportation propulsion—cars, trucks, off-highway vehicles, railroad, marine, motorcycles—as well as provide mechanical and electrical power for a wide range of large and small applications. The two dominant types of internal combustion engines are spark-ignition and diesel. The amount and composition of the emissions exhausted from these engines depend on the details of the processes that occur within the engine during operation, the characteristics of the fuel used, and the type of emissions control system used.
For diesel engines, the main pollutants of concern are nitrogen oxides (NOx) and particulate matter (PM). The latter is composed of black smoke (soot), sulfates generated by the sulfur in fuel, and organic components of unburned fuel and lubricating oil.
To reduce NOx, one approach is the use of NOx reduction catalysts, such as lean NOx traps (LNTs), also referred to as NOx absorber catalysts (NACs). To reduce PM, one approach is the use of various types of diesel particulate filters (DPFs). These devices may be used alone or together, with either or both being used downstream of the engine, in the exhaust line. Both operate in repeated loading and regeneration cycles.
A characteristic of Nox reduction catalysts is that they accumulate NOx and must be periodically regenerated. For regeneration, the oxygen content in the exhaust is reduced. This reduced oxygen content is usually combined with increased exhaust hydrocarbon content, to obtain a rich mixture needed for the NOx regeneration process.